During the warm weather months I ship plenty of fish - mostly fry and juvies - via USPS (Priority mostly). I have an oxygen tank and plenty of experience with bagging / packing fish for shipping. It's not that hard.
Winter and large fish are a whole different situation, especially anywhere near the holidays. There are a million things that can go wrong, about 4/5 of them entirely outside of your control. It's honestly just now worth it to me.
I very seldom have issues shipping fish via USPS. Whether I get turned away from shipping fish via my local FedEx and UPS locations is entirely based on who's working that day. Sometimes no problem at all. Other times I get a story about it being against the rules and that Fedex (or UPS) doesn't ship fish (even if I have a box labeled "live fish" that was shipped to me by FedEx or UPS). So I'm left scrambling to get to USPS in time.
I've also had bad experiences with shipping fish to people on MFK - scammers, time-wasters, people who want a million pictures, lowballers, people who dispute shipping chargers, etc. It's just not worth the time or effort.